1. Field of the Invention
The present invention relates to a liquid dispensing apparatus, and more particularly to a liquid dispensing apparatus having a liquid surface detection function suitable for sampling of a reagent, a sample, or a reaction solution of the reagent and the sample in an automated analyzer.
2. Description of the Related Art
An automated analyzer, such as a biochemistry analyzer, an immunological analyzer, etc., is provided with a dispensing apparatus which automatically performs suction and discharge (hereinafter abbreviated to dispensing) of a liquid sample and reagent from sample and reagent cassettes, respectively, to a vessel.
In particular, a biochemistry analyzer performs the steps of: dispensing a biological sample, such as blood, urine, etc., to a first or second cassette storing a sample; further dispensing a reagent from a third cassette storing a reagent to the second cassette into which the biological sample was dispensed; and measuring a color change produced in a mixture of the sample and the reagent by use of measurement means such as a photometer.
During sample and reagent dispensing operations, the leading end of a dispensing probe is immersed into a solution to be dispensed. The larger the depth (immersion depth) with which the leading end of the dispensing probe is immersed, then the larger the amount of solution which adheres to an outer wall of the dispensing probe, and the larger the contamination between different samples and reagents.
Then, in order to reduce the immersion depth of the dispensing probe as much as possible, such an operation control technique is commonly performed that includes the steps of: detecting a liquid surface in a cassette; stopping the lowering operation of the probe at a timing when the leading end of the probe has reached a position slightly below the liquid surface; and sucking a predetermined quantity of solution into the probe.
A technique for correctly detecting a liquid surface is important for such operation. Examples of a proposed technique for detecting a liquid surface includes a method of measuring a capacitance between the dispensing probe and the solution, a method of measuring pressure changes in the dispensing probe, and the like.
Analyzers using any of these methods are disclosed, for example, in JP-A-62-218818, JP-A-63-259420, JP-A-2-59619, and JP-A-8-114604.
Although the above-mentioned conventional methods use different liquid surface detection methods, each method detects whether or not a sample and a reagent solution exist at the start of suction and suction starts when the existence thereof is recognized or when the probe is moved to a position at which the existence thereof is assumed. Therefore, even if a necessary quantity of solution cannot be sucked or the sample or the reagent solution runs out during the suction operation, these states cannot be directly sensed.
As a method of detecting the above-mentioned suction state, a technique which detects whether or not a sample or a reagent solution is actually sucked by using a pressure sensor is known in JP-B-3-40343.
However, with such a method of using a pressure sensor, if the quantity of suction is very small, a sufficient pressure change cannot be obtained, thus making it difficult to correctly detect the above-mentioned state.